Spinal and bulbar muscular atrophy (SBMA) is an X-linked recessive and late-onset slow progressive lower motor neuron disease (Sobue et al., 1989; and Kennedy et al., 1968). Major symptoms are lowering of muscular power (weakness) and muscular atrophy in proximal limbs as well as bulbar palsy. Sensory nerve involvement, mainly deep sense impairment, is also found. As concomitant symptoms, gynecomastia is found at high rate, and hepatic dysfunction, glucose tolerance disorder, hyperlipidemia, hypertension, and the like, are often found. Also feminized skin, testicular atrophy, infertility and impotence are sometimes found. Female carriers are usually asymptomatic, although some express tremorof fingers, muscle cramp, mild increase in CK value, etc. (Sobue et al., 1993). A specific treatment for SBMA has not been established. Testosterone may be used as a symptomatic treatment, however, its effect is poor and the efficacy of long-term use has not been confirmed.
Pathologic conditions of SBMA include degeneration, or loss, of neurons in the ventral horn, facial nucleus and hypoglossal nucleus, which is caused by the abnormal expansion of CAG repeat in the first exon of the androgen receptor (AR) (La Spada et al., 1991). Normal number of the CAG repeats in AR gene is about 12 to 34, while that of SBMA patients is expanded to about 40 to 62. Therefore, SBMA is called polyglutamine disease along with Huntington's disease (HD), spinocerebellar ataxias, etc. These diseases share several pathologic conditions such as anticipation and variation in the number of CAG repeats (somatic mosaicism) (Tanaka et al., 1999), selective impairment of neuronal tissue. Furthermore) similar to the other polyglutamine diseases, SBMA also shows a negative correlation between the number of CAG repeats and age at onset of lowering of muscular power, and a positive correlation between the number of CAG repeats and the disease severity adjusted by age (Doyu et al., 1992).
Pathologically, nuclear inclusions stained with an anti-polyglutamine antibody and an anti-androgen receptor antibody are observed in the ventral horn, motor nucleus of lower cranial nerves, kidney, testis, skin, etc. (Li et al., 1998a; and Li et al., 1998b). Nuclear inclusion is among pathological characteristics of polyglutamine diseases and is thought to be involved in pathologic conditions of the disease (Zoghbi and Orr, 2000, and Paulson, 2000). However, the importance of nuclear inclusion in the pathophysiology of polyglutamine diseases has not been established. Some views that nuclear inclusion reflects a cellular protective mechanism for protecting the neuronal cell from the toxicity of the polyglutamine tract. On the other hand, many studies have suggested that, in almost all the polyglutamine diseases, nuclear translocation of mutant protein containing the polyglutamine tract is essential to the development of pathologic conditions (Klement et al., 1998, and Saudou et al., 1998). Unlike other polyglutamine diseases, SBMA is unique in that intracellular distribution of mutant protein changes due to the effect of the specific ligand. That is to say, it is known that AR, that is, a causative protein of SBMA, usually exists in cytoplasm as a complex in an inactivated state, and is transferred into the nucleus when it is bonded to testosterone that is a ligand (Zhou et al., 1994).
A toxic gain of function has been considered the main stream of the pathophysiology in polyQ diseases, but a loss of function of mutant proteins may also play a role (Zoghbi and Orr, 2000, Rubinsztein 2002). Although the expansion of polyQ tract in AR deteriorates the transcriptional activities of AR, and affects its interaction with other transcriptional factors and activators (Mhatre et al., 1993, Kazemi-Esfarjani et al., 1995, Chamberlain et al., 1994, Nakajima et al., 1996), the neurologic impairment in SBMA can not be attributed to the loss of AR function (Maclean et al., 1995, McPhaul et al., 1993), a reason why testosterone shows insufficient and transient effects when used as a therapeutic agent for SBMA (Danek et al., 1994, Goldenberg et al., 1996, Neuschmid-Kaspar et al., 1996).
There have been no substantially effective therapeutic approaches to the polyQ diseases. In a Tg mouse model of HD, expression of a dominant-negative caspase-1 mutant extended survival and delays the appearance of neuronal inclusions (Ona et al., 1999). Inhibition of mutant gene expression demonstrated the reversibility of phenotypic progression in a Tg mouse model of HD (Yamamoto et al., 2000). However, these gene modulations can not be directly applied clinically. Transglutaminase inhibitors suppressed aggregate formation and apoptosis in a cell model of DRPLA (Igarashi et al., 1998) and prolonged survival in a Tg mouse model of HD (Karpuj et al., 2002). An in vitro model of HD showed inhibition of huntingtin fibrillogenesis by specific antibodies and small molecules (Heiser et al., 2000). Creatine increased survival and delayed motor symptoms in a Tg mouse model of HD (Andreassen et al., 2001). Over-expression of molecular chaperone HSP70 demonstrated preventive effects in a Drosophila model of Machado-Joseph disease (Warrick et al., 1999) and SCA1 cell and Tg mouse models (Cummings et al., 1998, Cummings et al., 2001). HSP70 and HSP40 showed preventive effects also in a SBMA cell model (Kobayashi et al., 2000). These and other therapeutic approaches have, yet, remained insufficient or minimal in prevention of phenotypic expression and progression. Recently reported histone deacetylase inhibitors in a Drosophila model could be a promising candidate therapy for polyQ diseases (Steffan et al., 2001), but their therapeutic efficacy should be assessed in a Tg mouse model.
Meanwhile, generation of mouse models of SBMA and development of treatment method using the same have been attempted and some have been reported so far. Since no phenotypes were found in mice including a human androgen receptor gene (AR gene) having 45 or 66 CAG repeats, which are the same level as those of SBMA (Bingham et al., 1995, La Spada et al., 1998, and Merry et al., 1996), a Tg mouse having a truncated AR gene or a strong promoter have been developed for the purpose of obtaining mice showing symptoms. Mice expressing only 239CAG repeats under promoters of AR genes (Adachi et al., 2001) and mice in which a truncated AR gene having 112 CAG repeats is introduced (Abel et al., 2001) showed nuclear inclusions in the spinal motor neuron, in addition to symptoms such as motor weakness, weight loss, short life-span, etc.
As mentioned above, SBMA shows a remarkable sexual difference, that is, most patients are men and even if women have genetic abnormality, symptoms hardly appear. However, in the previous transgenic mice into which a truncated AR gene is introduced, such a sexual difference in symptoms is not observed (Adachi et al., 2001, and Abel et al., 2001). It is thought that no sexual difference in symptoms is found because transgenes of these mice do not have a full length and do not have the sequence encoding a biding site to ligand (testosterone). There was in the past only one SBMA Tg mouse model with a full-length human AR, which showed motor impairment, however, no significant sexual difference in symptoms has not been reported (Morrison et al., 2000).